1. Field of the Invention
The present invention relates to a resist protective film composition for forming a protective film on a photoresist film for the purpose of protecting the photoresist film in photolithography for micropatterning processes in manufacturing processes of semiconductor devices and so on, for example, in the liquid immersion photolithography in which ArF excimer laser having a wavelength of 193 nm is used as a light source and liquid such as water is inserted in a gap between a projection lens. and a substrate; and to a patterning process using the resist protective film composition.
2. Description of the Related Art
There has been a demand to achieve a finer pattern rule along with a tendency in which integration and speed of LSIs have become higher in recent years. And in the optical exposure, which is used as a general technique at present, resolution has almost reached its inherent limit derived from a wavelength of a light source.
The optical exposure has been widely used so far using g line (436 nm) or i line (365 nm) of a mercury-vapor lamp as a light source when a resist pattern is formed. Then it has been recognized that a method of using an exposure light with a shorter wavelength is effective as a means for achieving a further finer pattern. For this reason, KrF excimer laser with a shorter wavelength of 248 nm has been used as an exposure light source instead of i line (365 nm) for mass-production process of a 64 M bit (a processing dimension of 0.25 μm or less) DRAM (dynamic random access memory) and beyond.
However, in order to manufacture DRAM with an integration of 256M, 1 G or more which requires a still finer processing techniques (a processing dimension of 0.2 μm or less), it is recognized that a light source with far shorter wavelength is needed. Therefore, the photolithography using ArF excimer laser (193 nm) has been earnestly examined since about 10 years ago.
At first, it was planned to apply ArF lithography to fabrication of 180 nm node devices and beyond. However, application of KrF excimer lithography has been extended up to mass-production of 130 nm node devices, and ArF lithography is applied on a full-scale basis to fabrication of 90 nm node devices and beyond. Furthermore, fabrication of 65 nm node devices has been examined with combination of ArF lithography and a lens having an enhanced NA of 0.9.
As for fabrication of the next 45 nm node devices, shorter exposure wavelength has been achieved, and F2 lithography at a wavelength of 157 nm was suggested to be a possible choice. However, it was suggested to postpone introduction of F2 lithography and early introduction of ArF liquid immersion lithography due to various problems such as increase in cost by using large amounts of expensive CaF2 single crystals for a projection lens; necessary change of optical system associated with introduction of a hard pellicle because a soft pellicle has extremely low durability; and decrease of etching resistance of a resist (See Proc. SPIE Vol. 4690 xxix).
In the ArF liquid immersion lithography, it has been suggested to fill a gap between a projection lens and a substrate with water. Water has an index of refraction of 1.44 with 193 nm light, and a pattern can be formed even with using a lens having an NA of 1.0 or more. In theory, NA can be increased up to 1.44. Resolution is enhanced by increment of NA. It is shown that combination of a lens having an NA of 1.2 or more and ultra resolution techniques can realize fabrication of 45 nm node devices (See Proc. SPIE Vol. 5040 p 724).
As for the ArF liquid immersion lithography, various problems were pointed out due to the presence of water on a photoresist film. That is, the problems include pattern deformation due to leaching of generated acid and an amine compound added to the resist film as a quencher to water; pattern collapse due to water swelling of a photoresist film; and the like. Then it has been suggested that a protective film is placed between the resist film and water (See 2nd Immersion Work Shop, Jul. 11, 2003, Resist and Cover Material Investigation for Immersion Lithography).
The protective film that is formed on a photoresist film has been investigated as an antireflection film. For example, Japanese Unexamined Patent Application Publication No. 62-62520, No. 62-62521, and No. 60-38821 disclose the ARCOR (antireflective coating on resist) method. The ARCOR method is a method including a step of forming a transparent antireflective coating on a resist film and removing the antireflective coating after exposure. The ARCOR method is such a convenient method and provides fine, highly accurate pattern having high positioning accuracy. Use of perfluoro alkyl compounds having a low refractive index such as perfluoro alkyl polyethers or perfluoro alkyl amines for forming an antireflective coating reduces remarkably reflection at the resist/antireflective coating interface, thereby enhancing dimensional accuracy. As examples of fluorinated compositions, other than the compositions mentioned above, there has been suggested amorphous polymers such as perfluoro (2,2-dimethyl-1,3-dioxol)-tetrafluoroethylene copolymer, cyclized polymers of perfluoro (allyl vinyl ether) or perfluoro butenyl vinyl ether, and the like, which is disclosed in Japanese Unexamined Patent Application Publication No. 05-74700.
However, the perfluoro alkyl compounds have a low compatibility with organic compounds, and thus flons and the like are used as a diluent for controlling applied film thickness. And it is a known fact that use of flons is now perceived as a problem in view of environmental protection. Furthermore, the compound has a problem in forming a uniform film, and the compound is not suitable for antireflective coatings. In addition, the antireflective coating has to be stripped with flons prior to developing of a photoresist film. Therefore, there are many practical detriments of involving additional installation of system for stripping the antireflective coating to conventional equipment, mounting costs of flon solvents, and the like.
When the antireflective coating is stripped without any additional installation to conventional equipment, most desirable is to strip the antireflective coating with a developing unit. Solutions that are used in a developing unit for photoresist films are aqueous alkaline solutions used as developers and pure water used as a rinsing solution. And thus, a desirable antireflective coating composition can be stripped with the aqueous alkaline solutions or pure water. Therefore, there have been proposed many water-soluble antireflective coating compositions and patterning processes using the compositions (For example, see Japanese Unexamined Patent Application Publication No. 06-273926 and Japanese Patent Publication No. 2803549).
However, water-soluble protective films cannot be used for liquid immersion lithography because such films dissolve in water during exposure. On the other hand, water-insoluble fluorinated polymers has problems of requiring a special flon stripping agent and a stripping cup intended only for flon solvents. Then there has been demanded a water-insoluble resist protective film that can be stripped easily.
There has been proposed a top coat based on hexafluoro alcohol pendant methacrylate that is soluble in a developer (For example, see J. Photopolymer Sci. and Technol. Vol. 18 No. 5 p. 615 (2005). The top coat has a high Tg of 150 degrees C., a high alkali solubility, and is suitably used with photoresist films.
By the way, in the above cases, there can occur problems depending on the types of photoresist films that when a protective film is formed on a photoresist film, film loss occurs in the surface portions of developed photoresist film and the photoresist film has a rounded top shape. In this case, rectangular and excellent resist patterns cannot be obtained. Therefore, there has been demanded a resist protective film composition that more certainly provides rectangular and excellent patterns.